1. Field of the Invention
The present invention relates to an apparatus for controlling the supply of electric power to an electrically heated catalyst. More particularly, the invention relates to an apparatus for controlling the supply of electric power to an electrically heated catalyst attached to the exhaust gas passage of an automobile in order to heat the electrically heated catalyst to promote the activation of the catalyst when it is cold at the time of cold starting of an internal combustion engine.
2. Description of the Related Art
Exhaust gases emitted from an internal combustion engine of a vehicle contain such harmful substances as HC (hydrocarbons) and NOx (nitrogen oxides). In an exhaust gas passage of an internal combustion engine is usually provided a catalytic converter which is a device for removing impurities contained in the exhaust gases. However, it has been known that when the temperature of the catalyst is low (in an inactive state), the three-way catalyst used in the catalytic converter works to remove only some of the harmful substances contained in the exhaust gases. Therefore, when the catalytic converter is in the inactive state after cold starting of the internal combustion engine, the exhaust gases cannot be cleaned to a sufficient degree.
There has been proposed a device for cleaning exhaust gases which is installed in the exhaust gas passage on the upstream side of the catalytic converter, the device incorporating an electrically heated second catalytic converter (EHC: electrically heated catalyst) carrying an oxidizing catalyst and having an electric heater, so that when the first catalytic converter is still inactive, the second catalytic converter is electrically heated to activate the oxidizing catalyst and to promote the removal of HC.
Electric power must be supplied to the second catalytic converter (hereinafter referred to as electrically heated catalyst) only until the catalytic converter is activated. The electric power supplied to the electrically heated catalyst can be controlled by detecting the temperature of the electrically heated catalyst. A temperature sensor is usually required to directly detect the temperature of the electrically heated catalyst. However, the electric resistance (which rises with an increase in the temperature) of the electrically heated catalyst which is nearly equal to the catalyst temperature can be detected, without using the temperature sensor, to thereby indirectly detect the catalyst temperature of the electrically heated catalyst.
Japanese Unexamined Patent Publication (Kokai) No. 5-1526 discloses technology according to which the electric resistance of the electrically heated catalyst is detected and the amount of electric power supplied from the battery is controlled by feedback depending upon the electric resistance that is detected. That is, according to technology disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-1526, the feedback control is effected based upon the electric resistance of the electrically heated catalyst from the moment of starting the supply of electric power to the electrically heated catalyst, and the electric resistance is detected at all times.
However, it is difficult to correctly measure the electric resistance of the electrically heated catalyst in a manner as taught by Japanese Unexamined Patent Publication (Kokai) No. 5-1526. In particular, it is difficult to correctly measure the electric resistance of the electrically heated catalyst at the time of starting the supply of electric power, and the control performance is impaired.